This invention relates to transducer apparatus and, more particularly, to a phase-sensitive transducer apparatus of the type comprising first and second relatively movable members wherein the first relatively movable member has a plurality of windings and the second relatively movable member has a winding, and means for applying a first input signal to one of the windings of the first relatively movably member and a second input signal to another of the windings of the first relatively movable member, an output signal being developed, as by induction, on the winding of the second relatively movable member. In this type of transducer apparatus, the first and second input signals are generally sinusoidal in nature of substantially identical frequency and peak amplitude and are phase-displaced by a predetermined amount (e.g. 90.degree.), and the output signal is substantially constant in peak amplitude and variable in phase during relative movement of the first and second relatively movable members.
Phase-sensitive transducer apparatus of the above type have been used in systems where it is desired to sense and record and/or control movement of a movable element. This is generally accomplished by kinematically coupling one of the two relatively movable members, above defined, to the movable element in order that they are able to move in synchronism. The first relatively movable member may remain fixed and be considered a stator, in the case of a rotary transducer, or a scale, in the case of a linear transducer. Likewise, the second relatively movable member be coupled to the movable element and be considered a rotor, in the case of a rotary transducer, or a slider, in the case of a linear transducer.
Assuming for the purpose of future explanation that the transducer is of the linear type, the output signal from the single slider winding will be phase-modulated in the sense that its peak amplitude will remain constant and its phase will change during movement of the slider relative to the scale. Then, by appropriately demodulating the output signal, a position signal may be derived that is periodic in nature in response to slider movement, wherein each new period of the position signal is indicative of movement of the slider and thus corresponding movement of the movable element. A phase-sensitive transducer apparatus of this general type as used in a position measuring system is disclosed in U.S. Pat. No. 3,191,010.
Phase-sensitive transducer apparatus can also be used in conjunction with a servo control system to control the direction and speed of movement of a movable element. In such context, a means would be provided for deriving a position signal from the slider output signal which alternates during movement of the slider and thus movable element relative to the scale between predetermined first and second voltage levels. In many such servo control systems, velocity information is derived from the position signal, as by differentiation techniques, and both velocity information and position information are used in controlling the direction and speed of movement of a movable element. An example of one such servo system is disclosed in U.S. Pat. No. 3,839,665.
In most contemporary servo systems utilizing a position signal as derived from a transducer apparatus, whether phase or amplitude sensitive, movement of the controlled movable element is generally detected by sensing "zero-crossings" of the position signal. By the term "zero-crossings" in its broader context, it is meant those portions of the position signal which ideally cross an imaginary line half-way between the positive-going and negative-going peaks. In a phase-sensitive transducer apparatus, the position signal would normally alternate between ground or zero voltage and a positive (or negative) peak voltage, thereby making the "zero-crossings" equal to one-half the peak voltage. Consequently, it would be necessary, if a phase-sensitive transducer of this type were used in a servo control system employing "zero-crossing" detection, to carefully adjust the detection system for one-half peak voltage detection. It should be clear that any amplitude fluctuations and offset errors that occur in the position signal would make accurate "zero-crossing" detection extremely difficult, thereby leading to possible servo errors.
Copending U.S. application Ser. No. 670,463 filed on Mar. 25, 1976 now U.S. Pat. No. 4,059,789 in the name of Kenneth W. Cocksedge and assigned to the assignee of the present invention, discloses an improved phase-sensitive transducer apparatus that is capable of offsetting the initially generated position signal by an amount equal to one-half the peak amplitude thereof in order for it to be substantially balanced about a predetermined reference potential, such a ground (zero) voltage. Generally speaking, this is accomplished by using a comparator to compare the initially generated position signal with a d-c reference signal that has a voltage level substantially equal to one-half the peak amplitude of the position signal. The d-c reference signal is derived from the same d-c power supply that is used to establish the peak-amplitude level of the position signal.
As disclosed in the aforesaid application Ser. No. 670,463, the generated position signal is derived by obtaining, through filtering techniques, the d-c average of a pulse signal of constant peak amplitude and variable pulse width that is generated as the slider is moved relative to the scale. Although the phase-sensitive transducer apparatus disclosed in that application has worked quite well, it will be appreciated that errors might occur if the pulses vary substantially from a truly ideal waveform, e.g. they have different rise times than fall times or they do not go all the way to ground due to saturating offsets or the like. In either of these events, the d-c average voltage level of the pulse signal might be different than one-half the peak amplitude at a time when this relationship would normally be true if the pulse signal were idealized in format. More specifically, the d-c average of the pulse signal defines a locus of points during relative movement of the slider and scale, which locus of points constitute the initially generated position signal. Normally, the pulse signal would be expected to have a d-c average voltage level equal to one-half the peak voltage when at a 50% duty cycle. However, if the pulses are not ideal, the d-c average voltage at this duty cycle may be different than one-half the peak voltage.
Since the offsetting reference voltage level in the apparatus of application Ser. No. 670,463 is always equal to one-half the peak voltage, if the pulse signal is not ideal, when it is at a 50% duty cycle the d-c average thereof may not equal one-half the peak voltage. Consequently, when the position signal is offset by one-half the peak voltage, the points on the offset position signal corresponding to a 50% duty cycle of the pulse signal from which the position signal is derived may not occur at a zero voltage level, i.e. the desired condition for truly accurate "zero-crossing" detection. Accurate "zero-crossing" detection is especially important in the case of disk drives, where the zero voltage points of the position signal may be used to define the centers of tracks on the disk.
It would be desirable, therefore, to provide a phase-sensitive transducer apparatus with an improved signal offset means that is substantially insensitive to non-idealities of the waveform of any pulse signal or signals from which the position signal may be derived.